Huawei has launched E303 modem in various countries, some are directly unlockable using the unbranded Huawei mobile partner like you can see in this article (Hutch E303 modem unlock solution). I mean to say that Huawei has launched E303 modem somewhere with un-customized firmware and on some places with customized firmware. Un-customized firmware can be unlocked directly as you seen in this article 
But, if your Huawei E303 modem comes with customized firmware then the above-mentioned solution will not work for you, and your Huawei E303 modem can not be unlocked.
For unlocking Huawei E303 customized firmware modem you need to first flash the modem with un-customized firmware and then you can follow the normal procedure which is available step by step below.

How to change the firmware of Huawei E303 customized modem?

1. Download all the software and firmware from the link which is provided at the end of the article.
2. Now run Universal Master code software, go to Huawei tab and generate flash code according to the IMEI for your modem.
3. Now run the download firmware file of Huawei E303 modem and when it asks for password then provide the flash code (which you generated earlier).
4. Now wait for Finish.
5. Now, close the software, dashboard or connection manager, if it opens automatically.
6. Run the CardLock_Unlock tool.
7. Now put your 8-digit unlock code into the unlock code box and click on OK.
8. Now you will get a message “Unlock Success”.
Note : Don’t enter any wrong unlock codes into your Huawei E303 modem, it has a new security. Comment below to get the unlock code.
Download Huawei E303 Firmware namal with dashboard
https://drive.google.com/open?id=10gSaZbRs0j092XaM8gvFFR0G4QTshx-w
 Download Huawei e303 Firmware HiLink
https://drive.google.com/open?id=1HPDJcut7-SLtEePH5VfKkX3kNGXXqOUs

Download Universal Master Code Tool
https://drive.google.com/file/d/0B0CjpB1t01b1MUdPNEIyM1NGSEU/view
 NOTE.    Password is (paid pro)  
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